«مَا فِي الدَّارِ إِلَّا زَيْدٌ»
Nobody is in the house except Zayd.
Here the predicate “being in the house” is assigned to Zayd alone. By contrast, “mā Zaydun illā qāʾim” selects “standing” as the predicate asserted of Zayd rather than a competing state.
Thus the same surface method—negation with exception—can restrict in different directions. Do not decide between “subject to predicate” and “predicate to subject” merely because both clauses contain mā and illā. Analyze what the exception retains and what the sentence predication concerns.